Route summarisation made simple (with a CCNA-style example)

Route summarisation (also called supernetting) means advertising one bigger route instead of many small ones. Smaller routing tables are faster to search and more stable, because a change inside the summary doesn't ripple through the whole network.

The example

You have four networks: 172.16.8.0/24, 172.16.9.0/24, 172.16.10.0/24 and 172.16.11.0/24. Only the third octet changes, so write it in binary:

8  = 0000 1000
9  = 0000 1001
10 = 0000 1010
11 = 0000 1011

The first six bits (000010) are the same in all four. Add the 16 bits of the first two octets and you get 22 matching bits, so the summary is 172.16.8.0/22.

Check the boundary

A /22 block is 4 networks of /24, and it must start on a multiple of 4 in the third octet. 8 is a multiple of 4, so the summary is exact. If your networks were 9 to 12 instead, a single /22 would not fit them, and the smallest single summary would cover extra space.

Watch out for extra space

A summary that covers addresses you don't own can attract traffic for networks that aren't yours, or hide a missing route. The CIDR / supernet calculator shows both the single summary and the exact list of routes, and tells you if the summary includes extra addresses.

Want more practice? Read IPv4 addressing and subnetting, then check your work with the subnet calculator.